1. Field of the Invention
The present invention relates to a transconductance amplifier, and more particularly to a complementary transconductance amplifier having a common mode feedback circuit and a method of amplifying the transconductance of the amplifier.
2. Description of Related Art
A transconductance amplifier is a circuit block that amplifies a voltage signal with a predetermined gain. Transconductance amplifiers may be used in applications such as Gm-C filters. The transconductance amplifier needs to have a high operating frequency, a low operating current, a high linearity and a broad tuning range. A typical transconductance amplifier has a structure of a differential amplifier and has a function of controlling a variable gain.
FIG. 1 is a circuit diagram illustrating an example of a conventional transconductance amplifier, and is disclosed in U.S. Pat. No. 6,271,688. Referring to FIG. 1, input voltages VIP and VIM are applied to the gates of NMOS transistors MN1 and MN2. An NMOS transistor MN3, coupled between the sources of the NMOS transistors MN1 and MN2, operates in a triode region and functions as a variable resistor. Transconductance (Gm), which is represented as IO/VI, is controlled by controlling a control voltage VC that is applied to a gate of the NMOS transistor MN3. The transconductance amplifier of FIG. 1 includes a common mode feedback circuit (CMFB) 12 to stabilize an output common mode voltage.
FIG. 2 is a circuit diagram illustrating another example of a conventional transconductance amplifier, and is disclosed in U.S. Pat. No. 5,332,937. Referring to FIG. 2, NMOS transistors MN4 and MN5, to which input voltages VIP and VIM are applied, operate in a triode region. Drain voltages of the NMOS transistors MN4 and MN5 are controlled in response to the control voltage VC, which is applied to bases of bipolar transistors Q1 and Q2. Therefore, transconductance (Gm) is controlled by way of controlling the control voltage VC.
FIG. 3 is a circuit diagram illustrating still another example of a conventional transconductance amplifier, and is described in Martinez et al., “A 60-mW 200-MHz continuous time seventh-order linear phase filter with on-chip automatic tuning system,” IEEE J. Solid-State Circuits, February 2003, Vol. 38, Issue 2, pp. 216-225. The transconductance amplifier of FIG. 3 has a complementary structure. A common mode feedback circuit 32 enables the output common mode voltage to have a constant value even though the transconductance (Gm) of FIG. 3 changes. Transconductance amplifiers having a complementary structure such as the transconductance amplifier of FIG. 3 have reduced current consumption and noise.
In the transconductance amplifier of FIG. 3, the transconductance of an N-type transconductor comprising NMOS transistors MN6 and MN7 is controlled by a control voltage VCN, and the transconductance of a P-type transconductor comprising PMOS transistors MP1 and MP2 is controlled by a control voltage VCP. In the transconductance amplifier of FIG. 3, one of the control voltages VCP and VCN needs to have a fixed value because the control voltages VCP and VCN are independent from each other. Accordingly, the transconductance (Gm) control range of the transconductance amplifier of FIG. 3 may be reduced by about 50% as compared with the transconductance amplifier of FIG. 1, and a degree of symmetry between the N-type transconductor and the P-type transconductor may be reduced.
The noise of the transconductance amplifier of FIG. 3 may be increased because of current sources IP1 and IP2, placed between the supply voltage VDD and the P-type transconductor, and current sources IN1 and IN2, placed between ground GND and the N-type transconductor.
The transconductance amplifier of FIG. 3 may have a lower linearity than the transconductance amplifier of FIG. 1 having NMOS transistors MN4 and MN5, which operate in a triode region, because the PMOS transistors MP1 and MP2 and the NMOS transistors MN1 and MN2 operate in a saturation region.
Accordingly, there is a need for a transconductance amplifier that has an increased linearity, a reduced operating current, an improved noise characteristic and a broad tuning range of transconductance (Gm).